42 
THE UMBO IN BIVALVES. 
siphons, etc., as is the case to a limited extent in the Naiads, the 
shells are then called Gaping. 
Fig. 101. — “Gaping" Bivalve. 
Afiodonta anatina (L.), 
Sandy bed of River Vare, Bramerton Wood-end, near Norwich, 
Collected by Rev. S. Spencer Pearce, IM.A. 
g. gape of shell on the posterior dorsal margin, for the protrusion of the siphons ; umbo or 
nucleus of the shell ; L ligament. 
The Umbo (see tig. 101) is the prominent part or apex of each 
valve near the hinge, it is formed around 
the embryonic shell, which is the nucleus 
or apex of the nmbo in each valve. They 
become wider apart with age and the 
growth of the shell, and are sometimes 
very different in character to the after- 
growth, as in Pisidl/nii //('iis/oiroiiuin and 
U/iio t/iui/diis, etc. In the different species 
of Piilo the peculiar and remarkable in- 
dnlation or sculpture of the iimbones is 
becoming recognized and acknowledged as an important and reliable 
character, not only for the purpose of accurate specific discrimination, 
but also, for the arrangement of the species into natural groups, as 
this peculiar feature of the genus is said to be remarkably constant 
and less subject to modification than testaceous characters usually 
Fig. 102. — Umboucs of I 'nio 
iuvtidus Phil, x 2, 
Evesham, 
Collected by Mr. J. Madison, 
Showing the nodules and 
nodular ridges, charactcri/ing 
the immature stage of giowlh. 
I' IG. hcnslcnvnnum (Shepp.), Fig. lOl . — Pisidiuin hc>islo7vanutn (Shepp.), 
young, magnified (after Jenyns). adult, posterior end, X 12, 
Showing the position and character of Pond, Cockerion, Darlington, 
the peculiar eave-like projection.s in the Collected by Mr. Charles Oldham, 
young shell. Showing the greatly altered relative position 
of the umbonal appendages at the maturity of 
the shell. 
are. The umbones are always turned towards the anterior end, and 
in those foreign species with somewhat coiled umbones, the left valves 
